Effects of an increase in reservoir drawdown rate on bank slope stability: A case study at the Three Gorges Reservoir, China

To increase the total power output of the Three Gorges Hydropower Station and generate greater economic benefits, it is necessary to reduce the time required for the water level of the Three Gorges Reservoir to drop, i.e., to increase the rate at which the water level drops. A rapid drop in the reservoir water level might result in a significant change in the reservoir bank environment, which can destabilize landslides that extend into the water, reactivate old landslide masses and form new landslide masses in the reservoir region. A typical slope that extends to the water in the Three Gorges Reservoir region was selected for study. We studied the geologic conditions of a selected landslide mass and developed an expression for the hydrodynamic pressure under the effects of groundwater within the slope area and an expression for the normal stress on a sliding surface under the effect of hydrodynamic pressures. In addition, the evolution of the stability of the slope (submerged to various depths) under the effects of the lowering of the reservoir level is investigated based on a strictly global analysis: (a) safety factor decreases about 14% as the reservoir level drops with rate of 0.6 m/d and about 16% with rate of 1.2 m/d at the fall of reservoir water level 30.0 m; (b) an increase in the drawdown rate of the Three Gorges Reservoir from 0.6 m/d to 1.2 m/d is unfavourable for the stability of the slope that extends into the water, but the extent of the decrease in safety factors resulting from this increase in rate is not significant.